1. Field of the Invention
The present invention relates to transmission-blocking vaccines against malaria and methods of preventing the transmission of the disease.
2. Background Information
The Plasmodium falciparum parasite is the major cause of malaria in humans. The life cycle of the parasite begins in man when young malarial parasites or "sporozoites" are injected into the bloodstream of a human by the mosquito. After injection the parasite localizes in liver cells from which after approximately one week, the parasites or "merozoites" are released into the bloodstream. The entry of the parasites into the bloodstream begins the "erythrocytic" phase. Each parasite enters the red blood cell in order to grow and develop. When the merozoite matures in the red blood cell, it is known as a trophozoite and schizont. A schizont is the stage when nuclear division occurs to form individual merozoites which are released to invade other red cells. After several schizogonic cycles, some parasites, instead of becoming schizonts through asexual reproduction, develop into large uninucleate parasites. These parasites undergo sexual development.
Sexual development of the malaria parasites involves the female or "macrogametocyte" and the male parasite or "microgametocyte." These gametocytes do not undergo any further development in man. Upon ingestion of the gametocytes into the mosquito, the complicated sexual cycle begins in the midgut of the mosquito. The red blood cells disintegrate in the midgut of the mosquito after 10 to 20 minutes. The microgametocyte continues to develop through exflagellation and releases 8 highly flagellated microgametes. Fertilization occurs with the fusion of the microgamete into a macrogamete. The fertilized parasite is known as a zygote that develops into an "ookinete." The ookinete penetrates the midgut wall of the mosquito and transforms into the oocyst within which many small sporozoites form. When the oocyst ruptures the sporozoites migrate to the salivary gland of the mosquito via the hemolymph. Once in the saliva of the mosquito, the parasite can be injected into a host.
Malaria vaccines are being developed against different stages in the parasite's life-cycle including the sporozoite, asexual erythrocyte, and sexual stage. Each development increases the opportunity to control malaria in the many diverse settings within which the disease occurs. Sporozoite vaccines would prevent mosquito-induced infections. First generation vaccines of this type have been tested in humans. Asexual erythrocytic stage vaccines would be useful in reducing the severity of the disease. Multiple candidate antigens have been cloned and tested in animals and in humans.
One type of vaccine being investigated to slow or reverse the worsening epidemic of malaria is a transmission-blocking vaccine [Miller et al., Science 234:1349 (1988)]. Transmission of Plasmodium falciparum from host to mosquito vector can be blocked by monoclonal antibodies against a 25 kDa sexual stage surface protein, Pfs25, expressed on zygotes and ookinetes [Vermeulen et al., J. Exp. Med. 162:1460 (1985)]. The gene encoding Pfs25 has been cloned [Kaslow et al., Nature 333:74 (1988)], and the deduced amino acid sequence revealed a striking feature, the presence of four tandem epidermal growth factor (EGF)-like domains. EGF-like domains are cysteine rich and depend on proper disulfide bond formation for structural integrity [Savage et al., J. Biol. Chem. 247:7612 (1972)]. Of the monoclonal antibodies previously known to block transmission, none recognize the reduced Pfs25 antigen [Vermeulen et al., J. Exp. Med. 162:1460 (1985) and Carter et al., Prog. Allergy 41:193 (1988)], suggesting that for at least some of the blocking epitopes, disulfide bonds are involved in creating proper conformation.
Before Pfs25 can be used as an effective transmission-blocking vaccine, peptides or recombinant DNA-derived Pfs25 molecules having the appropriate conformation for immunological activity in vivo must be produced in large quantities. In addition, adjuvant formulations must be developed which are suitable for use in human transmission-blocking vaccines.